Potentiometer



June s, 1937. l L M STEIN 2,083,408

POTENTIOMETER Filed May s, 1956 2 sheets-sheet 1 1759i, l? Y VC a. L'

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June 8,'1937". 1. M. STEIN ,2,083403 POTENTIOMETER Filed May8, 1956 i 2 Sheets-Sheet 2 Patented Janes, r1037 PATENT. oFI-lci-:

PoTENrIoMETEa Irving M. Stein, Philadelphia, Pa., 4assignor to Leeds & Northrup Company, Philadelphia, Pa., a corporation of Pennsylvania Application May 8, 1936, Serial No. 78,620

14 Claims.

f My invention relates to potentiometers-and particularly to potentiometers suited for high preclsion measurements involving use of thermocouples, precisionresistance thermometers, and the like, or for standard cell comparisons. 'In accordance with my invention, a small number of fixed 4main resistances, preferably of low ances, and the taps or junctions are connected to switch or dial points selectively engageable by a relatively movable switch contact connected to a source of standardized current; preferably comtaps to maintain constant, for the different set-l tings of the switch contacts, the resistance of the path traversed by the standard current. ,i Further in accordance with my invention, the auxiliary resistances of dii/I/erent groups are of the `saine magnitude and to obtain the desired ratios between the incremental changes of balancing vvoltage effected by adjustment of the different switch contacts, there is included in the shunt path for one or more of the main resistances an additional resistance of proper magnitude to leflect equality of the sums of the main resistances id theiri associated resistances connected thereeach of these additional resistances also serves to attenuate the effect upon its associated main resistance of any thermal electro-motive force developed in theshunt path.

e ,Further in accordance with my invention the main resistancesare enclosed 'in a common houspensating resistances are interposed in connec- 1100: 10: 1.

tions from the switch points to said `junctions or *fj may be of 11 ohms, 1.1 ohms, 0.11 ohm and 0.011

potentiometer system embodying the invention;

Fig. 1A is a detail view of the preferred disposition of the main coils within the thermal shield; j

Fig. 1B illustrates a simplified potentiometer 5 system having a vrange-changing arrangement;

Figs. 2, 3, 4 and 5 illustrate various uses of the potentiometer system;

Figs. 6 and 7 illustrate alternative switching or dial arrangements;

' Figs. 8 andSA illustrate uses of current-attenuating networks in a potentiometer system similar to Fig. 1.

In Fig. 1 is shown a iour-dialypotentiometer embodying the invention, though it is to be under- 15 stood the invention may be applied to potentiometers having a fewer or greater number of dials. The main resistances M I, M2, M3 and M4, corresponding in number to the number of places to which voltages are to beA measured, are connected 20 in series with each other, with a suitable indicating instrument, as galvanometer G, and the source E ofl voltage to be measured, for example,

a thermocouple. Preferably the values of the main resistances Ml to M4 are in the ratio 1000; 25

For example, resistances Ml to M4 ohm, respectively. These resistances are accurately calibrated and are made of conductor, as manganin, Therlo, o`r gold chromium having a 30 low temperature coefficient of resistance and lowv thermal electro-motive force against copper.

Preferably,: these resistances are enclosed in a housing or thermal shield H suitably constructed to maintain uniformity of temperature within the 35 enclosure. The housing H is preferably of metal, as copper or aluminum having high thermal conductivity so that no temperature gradient of appreciable magnitude can exist between one part of the housing and any other part; preferably, the 40 housing comprises two or more complete shells, one within the other, of high thermal conductivity with thermal insulation in thespace between the shells. With this arrangement, heat impinging on the outer shell will be distributed .45

` by the outer shell, attenuated by the insulation in passing therethrough to the inner shell, and if not cperfectly distributed, will be further ,distributedby the inner shell due to its vhigh thermal conductivity. All of the main resistancesand 50 .connections within the housing are, therefore, at the same temperature.

Each of the main resistances Ailo-M4 is-,Mfshunted by an associated auxlliary'resistance; /i

specifically, the resistances AI, A2, A3, A4 are 55 may comprise a plurality of serially-connected resistances.

Between the'contacts SI and S2 of the upper dials DI and D2'is connected a' suitable sourcek of current B, such as a storage battery, "Air- Cell, or other battery capable of supplying 'oonstant current for suitably long periods of time.

The resistance R in series with battery B is for adjusting the battery current to a desired standard value. from battery B, the contact VC is adjusted along the resistance VR to apredetermined position, as

read from scale US, corresponding to the voltage of the particular'standard cell SC. Switch GS is then closed in its upper position temporarily to connect the standard cell SC and gaivanometer VR and the resistances SR and Sr.

GI across the selected portion of the resistance The resistance R is then adjusted to vary the current from battery B until the voltage drop produced by the battery current across resistances SR, Sr and the portion of resistance VR between resistance Sr .and contact VC is equal to the voltage drop pro-v duced by the standard cell, as indicated by a null deflection of galvanometer'GI.

Between the movable switch contacts-S3 and S4 o f the'two lower dials D3, D4 is connected another suitable source of potentiometer current BI. Ihe current from battery BI is similarly standardized by adjusting the resistance RI until the voltage drop across the predetermined portion of resistance VRI and resistance SRI is equal to the voltage of the standard cell SC.

After the battery currents have been standard` ized, the switch GS may be opened.

For the values of resistance herein given, the battery circuit resistance of the upper and lower dial potentiometer circuits is approximately 500 ohms; for a nominal measuring range of 100,000 microvolts the resistancesAR, RI are adjusted so that the standardized current is 0.01 amperes. The voltage of each of batteries B and BI is between ilve and six volts. The correct poling of the batteries is indicated.

This circuit arrangement, which makes it unnecessaryto shield more than a few coils of a system equivalent to a six-dial potentiometer, introduces a secondary problem arising because even the best obtainablegrade of manganin wire has an appreciable temperature coefiicient ofresistance, for example, about 0.001per cent per degree centigrade. To hold the error of measurement fromthis source to not'more thanabout 0.001 per cent, the` standard cell coil` resistance Sr, SR is divided and that part o f it designated Vby sa 1s disposed within the housin H; the remainder of it,vdesig nated Sr, disposed outside of -the housing H, is of such value that the sum of Aits resistance and one-half of resistance VR is approximately equal to 0.5 per cent oi' the value of the total standard cell resistance (Sr-FSR).

With the particular values assumed for re' sistances MI and ARI, resistance Sr should, for

---perfectcompensation, be of such value as to place approximatelyio per cent ofthe total standard cell resistance outsideof housing H. How-.-

To standardize the current flowing ever, if this is done, some error Vwill result in dial D2 unless part of resistance AR2 is disposed within the shield. In practice it has been found the compensation afforded by the simpler arrangement shown'permits obtainment of highly precise measurements.

In determining the magnitude of voltage developed by the source E, the several dials are adjusted to produce a minimum or null deflection of galvanometer G; when the switch contact SI, for example, is on the point 0 of dial DI, none of the standard current flows through the resistance MI, as the switch contact SI is moved successively into engagement with the switch points 1 to 10, a greater proportion of the standardl ly, the adjustment of each of the other dial contacts S2, S3, hSteifects change in the proportion of the total standard current which iiows through the main resistances M2, M3, and Ml, respectively, and so varies the voltage drop across the corresponding main resistance produced by the selected fraction of thestandard current. The variation in the voltage drop across the main resistance is obtained without inclusion in the galvanometer circuit of contacts relatively adjustable for that purpose, thus avoiding errors due to variations in contact resistance and to the thermal electromotive forces at such contacts.

To avoid change inithe magnitude of the battery current for different dialsettings, compensating reslstances are connected between the switch points and the 4tapping points of the auidliary'resistanc'es and these compensating Y. ohms and` that the auxiliary resistance AI isA comprised of ten equal sections a of ohms each; the values of the compensating resistances C0 to CIB, respectively, should be 300 ohms,

200 ohms, 'ohms, 60 ohms, 20 ohms, 0 ohms,

0 ohms, 20 ohms, 60 ohms, 120 ohms and 200y ohms; only ive different sizes of resistances are used.

The auxiliary resistances and the compensating resistances are of conductor, as manganin, or Therlo, having 4low temperature coemcient of resistance-and a low-thermal electro-motive fome against copper; all of the resistance coils,

`except where otherwise speciied, areA of such material.

It is desirable from a manufacturing standpoint that all ofthe auxiliary resistances AI, A2, AI, Al be equal and that the compensating resistances be the same for all dials. To that end, the additional resistancesI4 ARI, AR2, AR3 and ARI are utilized; the magnitudes of these resistances are such .that'the sum of each of them and its associated main resistance is equal to a common predetermined value. In the spe-- ciilc example herein described, resistance ARI4 is 99 ohms, resistance AR2 is 108.9 ohms, resistance Am is 109.89 ohmaand resistance Am 1s 109.989 ohms; orotherwisev stated, the magnitude of each of' these additional resistances is 110 ohms minus the resistance of its associated main resistor. These additional resistancesalso serve to attenuate the eilect of any thermal electro-motivefforces that may be generated inl the shunt circuit-of each main coil, because they circuit to the resistance coil.

Theneutral potentiometer system described has several important features; ilrst, there are no switch contacts included in the main circuit and susceptible of introducing thermo-electric effects in to the galvanometer circuit; second, the resistance''f `the-'galva-nometer circuit is low and therefore conducive to high sensitivity of the galvanometer; third, the resistance of the galvanometer circuit is constant and, therefore, insures good damping of the galvanometer; fourth, the number of thermally-shielded resistances is small, avoiding the diiiculties arising when it is necessary to shield 4forty or flfty coils and to bring out the many leads therefor to proper terminals, fth, `the main resistances are of relatively low magnitude and, therefore, easier to calibrate and better suited to hold their calibration, and, sixth, the system is well adapted to potentiometers readable to live or more significant gures or places.

With the system as thus far described, the potentiometer is direct reading to four places. If desired, the galvanometer G may be used as a' deflection instrument in determining two additional places; or preferably, the galvanometer is used as a null'instrument and the last two places'may bedetermined by network L includance M5.

` ing ammeter A.

The resistance M5 of the network L is also preferably enclosed within the housing H and inserted into the galvanometer circuit. In the example given its magnitude should be .0l ohm; the meter -A'may have a maximum range of .001 ampere, corresponding to a maximum -of microvolts. The current through resistance M5 from battery B2 is adjustable by movement of Contact CP along the resistance PR. When contact CP is at thelower end of resistance PR, the current supplied by battery B2 to the 'circuit including the meter A and resistance M5 is a maximum, determined by the magnitude of 'resistance AT. lAs contact CP is moved to the upper end of resistance PR, the fraction of the total battery current flowing through M5 is reduced; the minimum value, when contact CP reaches the upper end of resistancePR, is zero.

Any thermal electromotive .forces in the circuit including` CP have no'effect upon the accuracy of the readings and are ineffective to produce any deflection of meter A when contact PR isin minimum' position. Any currents ilowing as a result of thermal electromotive forces in the loop including meter A and resistance M5 are greatly reduced by the resistance of PR which is large compared to theresistance of the rest of the loop.

O f the six resistances shown in Fig. 1 within thefthermal shield H, only five are connected in the lgalvanometer circuit and the exactpreferred arrangement vof these ve resistances is shown in Fig. 1A. The circuit 35, M5, 36 is preferably a single piece of manganin wire ofthe same characteristic used in coils MI, M2, M3, M4, a central portion being tapped oif to form resist- The exact resistance of portions 35, 36 is not of importance. With this arrangement, the only junctions of different metals, coppermanganin, directly in the circuit of the lsensitive galvanometer G, are the connection of lead 26 with coil M3, and the connection of lead 21 with coil Ml; these junctions are located near each other and near the central part of the housing H where the temperature is most uniform. The thermal electromotivel forces developed at these junctions aretherefore substantially equal and since the voltages of these couples are in opposition, the residual thermo electromotive force is practically zero. Furthermore, with the exact arrangement of leads shown, there can be no fall of potential in .the sensitive galvanometer circuit, due to current from batteries B and Bl, when the four dials are all set at zero.'

If desired, the system may be equipped with a range changing device of known'type to change the standardized current and, at the same time, tochange the point at which the current -in the network L enters the galvanometer circuit. However, in the system shown having four dials and a 100 division meter in the networkfL, a range changing device is not necessary. thoughit would vbe desirable for a potentiometer of the type disclosed having fewer dials. By way of illustration, Fig, 1B illustrates a two dial potentiometer utilizing the invention and provided with a range changing arrangement permitting the range tn be standardize the battery current.

. changed by a factor of ten without need to re- When switch member HL is thrown to the right,

the potentiometer resistance is shunted by resistances p1' and'Spr in. series; resistance pr is equal in magnitude to the potentiometer resist-v ance PR which, as above stated, is constant for the current through PR is ten times greater than when switch HL is thrown to the left.

Forv making temperature measurements with a thermocouple, such as E (Fig. l), a lrefer- 'ence junction RJ is provided. For precision measurements, one may use copper-constantenv couples; one lead 31 from the couple E is copper and the constantan lead 39 is joined'to a copper wire 38 within flask F to form the reference junction. The junctions at I, I while of like metal,

copper, should preferably be at the same temperature to avoid thermal electromotive forces; all leads, binding posts, etc. in the galvanometer circuit are copper to avoid thermal electromotive forces. To that same end, the switchesSG and K3 which are in circuit when making the nal balance of the potentiometer, are.A constructed of copper conducting parts to be substantially free of thermal lelectromotive forces, and in addition, are housed in a thermal shield Hz to insure substantial uniformity of the temperatures of all their parts. The junctions I, I may for like reasons also be included within housing H2. The switches K2, Kl, disposed-exteriorlyof housing H2, are used to connect different Values of resistance in circuit with the galvanometer to obtain different sensitivities during preliminary ba1ances;' ,they are not in circuit when nal balance is made and hence no error can arise because of any thermal electromotive forces these switches may develop. Within shield H2 is disposed the coil G3, of copper, which affords critical damping for galvanometer G when switch or key K3 is in the up position.' .'1

Coil T is a copper coil of very low resistance, by which a minute electromotive force from battery B3, which may be a single dry cell, may be introduced into the galvanometer circuit. With switch or key K3 in its "up position, the "zero" as will appear from the following explanationof the use of the reversing switch SG.

Operation of switch SG effects reversal of the connections of galvanometer lG; without effect upon the relative polarities of the thermocouple and potentiometer electromotive forces; if these Lelectromotive forces-are equal and opposite, the.

reversal by switch SG does not change the galvanometer indication. If there is an unbalance, moving switch SG from its normal or up position to its "down" position will reverse this unbalanced electromotive force as` applied to galvanometer G, and change the indication of galvanometer G by an amount which is twice as great as that produced by the unbalance itself. The reversing switch is not used until balance is rather closely approximated using keys KI, K2, K3, after which switchA or key K3 islocked or held down and final balance is obtained by adjustment of the potentiometer for the different positions of switch SG.' If, on operating switch SG, there is a residual deflection but this deilection is symmetrical with respect to the zero ofthe galvanometer, the adjustmentof resist ance `R3 is known to be correct and no residual thermal electromotive forces are present in' the galvanometer circuit; hence the deflection should be reduced to zero by further adjustment of the potentiometer dials or resistance PR. On the .35 other hand, if upon reversal of switch SG, equal and opposite deflections are not obtained, resistance R3 should be adjusted to obtain that end.

Since galvanometer Gi, used for standardizing v f the battery currents, is always operating in a circuit involving a voltagevv of the order oi.' one volt,

avoidance of residual thermal electromotivev forces in its circuit is not important. It is `therefore not necessary.to provide a thermal ,shield for any of the galvanometer keys k2. k2, ki.

5 Galvanometer GI may be relatively insensitive; it

`is more convenient to usea second relatively insensitive galvanometer with a short period than to switch the sensitive galvanometer G into the standardizing circuit, which would have the further disadvantage of requiring in the sensitive galvanometer circuit, additional switch contacts ,which would become possible sourcesof undesirable thermal electromotive forces.

The potentiometer may be used accurately to calibrate current-response instruments, for example, an ammeter, by measuring the voltage nais I, I ofv the potentiometer. After the voltage E2 across that fraction of resistance VD has been determined by manipulation of the potentiometer, as above described, the result is multiplied by the factor which expresses the ratio of the total resistance VD to the fractional resistance. y For making precision temperature measurements by the resistance variation method, `the -terminals I, I.of the potentiometer are` connected (Fig. 4) to a reversing switch ISwhich, for its lower position, permits accurate determination by the potentiometer of the voltage drop across the standard resistance RS.` in series with battery B5 and the coils TR, of the resistance thermometer. and which for its upper positionpermits determination `by the potentiometer of the voltage drop across the resistance thermometer. The resistance of the thermometercoil is then determinable from the equation all RS 'ES in which ES=voltage drop across RS RS=standvard resistance ETvoltage drop across TR TR=resistance of thermometer coil.

'I'he potentiometer may be used for making standard cell comparisons by connecting across its terminals I, I (Fig. 5.) two standard cells SCI, SC2 so poled that their voltages are in opposition. The potentiometer is then balanced as above described to measure diil'erence between the voltages of the two standard cells. Assuming the voltage of one cell to be known, the voltage of the second ist equal to the algebraic sum 'of the voltage of the first and the measured l -diilerenca Figs. 6 and 7 disclose arrangements of the main dial switches which reduce the number of required compensating coils as compared to Fig. 1. I-Iowever, they require a more expensive switch and introduce another sliding contact for each dial which is a disadvantage when attempting to limit the error due to variation in contact resistance to less than 0.001 per cent. 'I'he arrangements of Figs. 6 and'l aresimilar except that f in Fig. 6 the compensating coils C are in series,

whereas in Fig.- 7 they are in a "radial arrangement. 01' these two arrangements, that` oi' Fig'.

drop produced by flow of the meter lcurrent through a standard resistance RS. 'Referring' to Fig. 2, the resistance RS is connected to .a

o suitable source of current, as battery BI and the the source by a voltage-dividing` arrangement such asslown in Fig. 3. The source E2 voi' the voltage to be determined is connected across re'- sistance VD. and a predetermined fraction cf 75 the total resistanceVD is connected to the termi- Then, since the- 6 is preferable when the compensating coil resistance is high or must dissipate a substantial amount of energy. i

In the modiilcatio'n of Fig. 1, as

described, the main coils Mi, M2, M2, MI are of different values. They may, however, be of the same value without need to use different auxiliary resistances or to use diil'erent arrangements of lcompensating resistances for the different dials.

Referring to Fig. 8, the main coils M l, M2 are of equal resistance, for example, 11 ohms; the suxiliary resistances AI, A2 are of equal resistance, for example, each comprises ten sections each of 110 ohms resistance; the attenuating r ARI, AR2 are -of equal value, for example. 99 ohms; and the corresponding compensating reslstances of the two'dials are also equal ss dejscribed inA connection with Fig. 1.y To obtain the desired magnitudes of current through the main resistance M2 for the diilerent'settings of con tact SIjthereis providedthe attenuating' section AS hcomprising resistances X and Y: resistance x isconnected in shunt to auxiliary resistance A2 and resistance' Y is interposed, as shown,

resistance AR2 and a terminal of resistance A2. The values of resistances X and Y are so chosen that the current divides through the path X, and the path including Y, AR2 and M2 inthe ratio of 9 to 1, and are also of such values that the resistance of the network X, Y, ARZ, M2 as viewed from the terminalsI of the auxiliary resistance A2 is the same as the network ARI, MI as viewed from the terminals of auxiliary resistance AI; for the values above given for M2, A2, etc., resistance X should be 1222+ ohms and resistance Y should be 990 ohms. The ARZ and Y coils may of course be replaced by a single resistance equal to their combined resistance. Briey stated,- the network AS attenuates the current through main resistance M2 to one-tenth of the current through `main resistance MI.

To obtain the desired current through main resistance M4, assuming all main resistances are ofthe same value, three such current attenuating networks, AS, ASI and A52 are employed in cascade, as shown in Fig. 8A; for dial D3, two such attenuating networks may be employed in casstrument, a source of voltage to be measured, two

fixed resistances connected in series with each other, said instrument and said source of Voltage, two shunt paths each individual to one of said ixed resistances and each including an auxiliary resistance, a source of standard current, and means for connecting said source of standard current between selected points of said shunt paths.

2. A potentiometer comprising an indicating instrument, a source of voltage to be measured,

' two xed`resistances connected in series with each other, said instrument and said source of voltage, two shunt paths each individual to one of said fixed resistances and each including an auxiliary xed resistance, groups of switch points,l

each group being connected to taps along one of said second-namedresistances, a source of standard current, and switch contacts, each movable with respect to one of said groups of switch points, connected to opposite terminals of said source of standard current.

3. A potentiometer comprising an indicating instrument, a source of voltage to be measured, two xed resistances connected in series with each other,.said instrument and said source of voltage, a. fixed resistance in shunt to each of said`iirstnamed resistances, groups of switch points, each group being connected to taps along one of said second-named resistances, a source oi standard current, switch contacts, each movable with respect to one of said groups of switch points, connected to opposite .terminals of said source of vstandard current, and compensating resistances included in the connectionsto said switch points of such magnitudes as to maintain constant, for the diierent settings of said switch contacts, the resistance 'of the path traversed by the standard current.

4. A potentiometer comprising an indicating instrument, a source of voltage to be measured, two xed resistances of different magnitude connected in scope with the appendedy closed loops.

in series with each other, said instrument and said source of voltage, auxiliary resistances of equal magnitude each providing a path in shunt. to one of said rst-named resistances and forming a closed loop therewith, groups of switch points, each group being connected to taps along one of said auxiliary resistances, a source of standard current connected to switch contacts,

. each movable with respect to one of said groups of switch points, and an additional resistance connected between at leastone of said first-named resistances and the associated auxiliary resistance and having proper magnitude to effec-t'substantial equality of the'total resistances of said closed loops.

' 5. A potentiometer comprising an indicating instrument, a source of voltage tov be measured, two xed resistances of different magnitude connected in series with each other, said instrument, and said source of voltage, auxiliary'resistances of equal magnitude .each providing a path in shunt to one of said iirst-named resistances and forming a closed loop therewith, groups of switch points, each group being connected to taps along one of said auxiliary resistances, a source of standard current connected to switch contacts, veach movable with respect to one of said groups of switch points, compensating resistances included /in` the connections to the switch points of each group of proper magnitude to maintain constant for the different settings of said switch contacts, the resistance of the path traversed by the standard current,

the corresponding .compensating resistances forv -the diierent groups being of similar magnitude,

and an additional resistance connected between at least one of said first-named resistances and' its associated auxiliary resistance to effect subtantial equality of the total resistances of said 6. A potentiometer ,comprising an indicating instrument, a source of voltage to be measured, a small number of xed resistances connected in series with each other, said instrument and `said source of voltage, a housing for said resistances ensuring substantial uniformity of their temperatures, means for producing standard current, and means external to said housing for individually varying the voltage drop across each of saidresistances due to iiow of standard current therethrough.

'7. A potentiometer comprising an indicating instrument, a source of voltage to be measured,v

a small number of fixed resistances connected in series with each other, said instrument and said source of voltage, a housing for said resistances ensuring substantial uniformity of their temperatures, auxiliary, resistances each connected in shunt to one of said first-named resistances and disposed exteriorly of said housing, and means for connecting a standard Source'v of current between selected points of adjacent auxiliary resistances.

8.A potentiometer comprising an indicating instrument, a source of voltage to be measured, xed resistances connected in series with each other, said instrument and said source of voltage, a housingfor said resistances ensuring substantialequality of their temperatures, auxiliary resistances, disposed externally of said housing, each connected in shunt to one of said rstnamed resistances and forming therewith a closed loop, means for supply of standard currentv connected to adjustable contact means associated with said auxiliary resistances, and additional eilect upon said first-named resistance ot-any thermal electromotive forces developed in said closed loops.

9. A potentiometer comprising an indicating instrument, a source of voltage to be measured, xed resistances connected in series with each other, said instrument and said source oi voltage, a housing for said resistances ensuring substantial equality of their temperatures, auxiliary resistances, disposed externally of said housing, each connected in shunt to one of said iirstnamed resistances and forming therewith a closed.loop, means for supply of standard current connected to adjustable contact means associated with said auxiliary resistances, and additional resistances, disposed externally of said housing, each included in one of said loops to attenuate the eiect of any thermal electromotive force developed therein.V Y ,l

10. A potentiometer comprising an indicating instrument, a source of voltage to be measured, xed resistances connected in series with each other, said source and said instrument, a housing for said resistances ensuring substantial uniformity of their temperatures, means for supplying.` current to said resistances, and means for standardizingsaidcurrent including a resistance disposed in part within said housing.

11. A potentiometer comprising an indicating instrument, a source of 'voltage to b measured, a small number of fixed resistances connected in series with each other, saidinstrument and said source of voltage, a housing for said resistances ensuringsubstantial equality of their temperatures, auxiliary resistances each connected in shunt to one oi' said mst-named resistances and disposed exteriorly of said housing, groups of switch points, each group being connected to -taps along one of said auxiliary resistances, compensating resistances disposed exteriorly of said housing and included in the connections to said switch points, a source of standard current, and yswitch contacts for connecting said source of current betwenselected switch points of dierent groups thereof. l Y

resistances in said closed loops to attenuate the' 13. A potentiometer for determining the magnitude of a voltage to at least four signiiicantgures comprising an indicating instrument, four uxed main resistances theratios of whose magnitudes are powers of ten, means for connecting laid resistances in series with each other, said indicating instrument, and the source of voltage under measurement, auxiliary resistances oi equal magnitude each providinga path in shunt to one of said ilrst-named resistances and forming a closed loop therewith, and additional resistances in circuit with said rst-named resistances and said auxiliary resistances of magnitude to eiIect equality of the total resistances of said closed loops.

14. A potentiometer for directly measuring the magnitude of voltages of less than about 100,000 microvolts to at least four significant gures comprising an indicating instrument, (our xed resistances, the ratios of whose magnitudes are powers of ten, means for connecting said resistances in series with each other, said indicating instrument, and the source of voltage under measurementr auxiliary resistances of equal magnitude providing paths in shunt to each of said first-named being connectedv to divide one oi said auxiliary resistances into two equal sections, switch contacts for selectively engaging the switch points.v

of each group, and sources of standardised current, each connected between a pair of said switch contacts.

resistances, groups of switch points each group l mvING is. s'nmc. l 

